BOSS pedal, effect works, effect off no signal

[MoruyaGuitarist]

Hi All,
just wondering if anyone has had this with a BOSS pedal before ?
It's on a BOSS NS-2 but think a lot of the BOSS pedals use similar circuits. The pedal works fine when turned on, but there is no signal at all when turned off. It uses FET's and a flip flop circuit to control sending the effect to the output or sending the "Bypass" signal to the output. Have replaced some of the signal caps, don't have any FET's so will have to try to find something similar. Only have a mulitmeter and no signal generators / oscilloscope equiv otherwise would trace the signal around the circuit.

Probably not going to spend much time on it, just wondering if it's something that's happened with other people. Worst case will give it the "True Bypass" mod.   

[ElectricDruid]

If it works fine when it's on, it can pretty much only be Q6.

Q5 being permanently on would kill the signal for both Effected and Bypassed. Faults in the Q4 or Q3 buffers would also affect both Effected and Bypassed. The only other thing in the dry path is Q6.

Try shorting the drain and source of the Q6 FET and see if that brings the dry signal back to life. If it does, then we need to work out why that FET isn't working - a dead FET, or just not getting a working control signal to the Gate?

[PRR]

If it works fine when it's on, it can pretty much only be Q6.

C6 C8? It looks old enough to have dry caps, and old/cracked solder is universal.

For troubleshooting, any of these parts may shorted momentarily to see if signal passes; it will POP! like a firecracker so keep your amp turned low.

[ElectricDruid]

C6 C8? It looks old enough to have dry caps, and old/cracked solder is universal.

Yeah, fair point.

I suppose the best option is to audio probe along that dry path. Q4 emitter, C8, Q6, C6, Q3 emitter. The signal's got to go dead somewhere, and that'll tell us where to look for the problem.

[MoruyaGuitarist]

Thanks Guys !
I had already replaced C8 as it was a part I had lying around. Replacing Q6 was also my thought, C6 is the signal path for both effect on&off. Unfortunately I do not have any type of audio probe / scope for tracking the signal. Thanks, has got me thinking though. The LED does turn on and off so it only leaves Q6 and possibly D4. Can get a similar part locally, not exact but enough to get it working, would have to order the exact part/equivalent online.

Cheers,
Wal

[ElectricDruid]

An audio probe is simple. You should definitely try it:

http://diy-fever.com/misc/audio-probe/

Like this, you'll be able to see how far the signal gets. That'll give you a good idea where the problem is. Don't necessarily assume it's the components - like PRR said, it might be a cracked joint or a damaged track, in which case a touch with a bit of new solder is all it needs.

[Locrian99]

My audio probe is literally a piece of solid core wire attached to a cap covered in heat shrink attached to another bit of wire and I just plug it into the speaking terminal in my test box.   I guess if you don't have a test box set up you need the jack but invaluable simple tool.  I just have my guitar on a stand and smack the low e with one hand while I poke. 

[ElectricDruid]

I just have my guitar on a stand and smack the low e with one hand while I poke.

I have a recording of some strumming on my phone and plug that in! Anything works, as long as it's sound!

[MoruyaGuitarist]

Thanks, will give it a try !
I thought it would be simple, guess it was more a case of laziness. Very long time ago in Technical college we had all the gizmos, Lab regulated power supplies, Signal generators and CRO's (yes I'm that old there were a few actual cathode ray oscilloscopes around). 

[GibsonGM]

I have 2 probes.   I had a DMM where one lead died, so I used the other for this purpose, incl. a jack on the end.   It COULD be just the cap and 2 jumpers to a guitar cord, if one needed to do it that way.

The one built into my test box is a piece of stranded wire, well-tinned at the top 2", and threaded up an old pen, then filled with epoxy. The cap is internal to the box.  I run a little sine oscillator into the ckt to generate a signal for testing.   There is an LED on the "Probe/Amp" switch as well, to let me know I"m in 'probe mode', as no audio will be heard UNLESS the probe is connected to something (which has screwed me up a time or two Ha Ha).

Some time I want to do some switching inside, mount a digital panel meter, and use it as a voltage probe too!
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Good luck Moruya, and many of us still have CROs that we use often, and enjoy very much!